1. Technical Field
The present invention relates to an optical position detection device for optically detecting the position of a target object, and a display system with an input function provided with the optical position detection device.
2. Related Art
As the optical position detection device for optically detecting the position of the target object, there is proposed a device having a plurality of point light sources disposed at respective positions distant from each other, in which detection lights reflected by the target object are transmitted through a light transmissive member and then detected by a light receiving section when emitting the detection lights respectively from the plurality of point light sources toward the target object via the light transmissive member (see, e.g., JP-T-2003-534554). Further, there are also proposed optical position detection devices using a method of emitting the detection lights, which are emitted respectively from a plurality of point light sources, via a light guide plate, and then detecting the detection lights reflected by the target object with a light receiving section (see, e.g., JP-A-2010-127671 (Document 2) and JP-A-2009-295318 (Document 3)).
In such optical position detection devices, one light receiving element is used as the light receiving section, and the position of the target object is detected based on the result of the comparison between an output from the light receiving element when lighting some of a plurality of point light sources and an output from the light receiving element when lighting some other of the plurality of point light sources.
However, since the optical position detection device described in Document 1 uses the detection lights emitted from the point light sources, the irradiation range itself of the detection light is narrow. Therefore, there is a problem that the range in which the position of the target object can be detected is narrow. Further, since the optical position detection devices described in Document 2 and 3 each form a light intensity distribution using the light guide plate, the range in which the detection is possible is limited by the size of the light guide plate. Therefore, there is a problem that the range in which the position of the target object can be detected is narrow.
Therefore, the inventors of the invention have studied a method of detecting the position of the target object on a virtual plane using the detection light emitted from a light source device along the virtual plane. For example, as in an optical position detection device shown in FIG. 16, the detection light L2 is emitted from the light source section 12 along an X-Y plane, and at the same time, the emission intensity of the detection light L2 is reduced in a direction from one side toward the other side of a space (a detection space 10R) in which the detection light L2 is emitted as indicated by the diameter of the circular arc L21 in a first lighting operation in a first period. Further, in a second lighting operation in a second period not overlapping the first period, the emission intensity of the detection light L2 is reduced in the direction from the other side toward the one side as indicated by the diameter of the circular arc L22. Then, the position of the target object Ob is detected based on the result of the comparison between the result of receiving the detection light (reflected light L3), which is reflected by the target object Ob in the first period, by the light receiving element 130 and the result of receiving the detection light (the reflected light L3), which is reflected by the target object Ob in the second period, by the light receiving element 130. According to such a configuration, since the detection light L2 is emitted along a coordinate plane (the virtual plane) for detecting the target object Ob, there is an advantage that the detection space 10R is large.
However, in either of the configuration explained with reference to FIG. 16 and the configurations described in Documents 1 through 3, there is a problem that the detection error due to the directivity in sensitivity of the light receiving element 130 occurs. Further, since a part of the detection light L2 reflected by an object other than the target object Ob enters the light receiving element 130 when emitting the detection light L2, if the target object Ob is located in the direction tilted at a large angle with the normal direction of a light receiving surface 130a, there is also a problem that the proportion of the output component due to the detection light L2, which is reflected by the object other than the target object Ob and then enters the light receiving element 130, increases to thereby degrade the detection accuracy.
Specifically, as shown in FIG. 17A, the light receiving element 130 is provided with an element main body 130b having a rectangular solid shape as a whole, and terminals 130c projecting from both ends of the element main body 130b, and the element main body 130b is provided with the light receiving surface 130a on one surface side. The light receiving sensitivity of such a light receiving element 130 has an incident angle dependency (the directivity in sensitivity) shown in FIG. 17B, and has a sensitivity peak direction in the normal direction with respect to the light receiving surface 130a. Further, as is understood from FIG. 17B, the sensitivity of the light receiving element 130 is degraded to be less than a half of the sensitivity peak value if the incident angle of the detection light is tilted equal to or greater than 60° from the normal direction with respect to the light receiving surface 130a, and if the incident angle of the detection light is tilted equal to or greater than 90° from normal direction with respect to the light receiving surface 130a, the sensitivity becomes 0. Therefore, if the incident angle of the detection light is significantly tilted with respect to the normal direction to the light receiving surface 130a, the level of the signal output from the light receiving element 130 is lowered, and thus the detection accuracy is degraded.
Specifically, when performing the first lighting operation and the second lighting operation while varying the angular position of the target object Ob, the output signal from the light receiving section 13 results in what is shown in FIG. 18A. In FIG. 18A, the lateral axis represents the angular position of the target object Ob assuming that the direction corresponding to the one side (the other side X2 in the X-axis direction) in the emission space of the detection light L2 is 0°. Further, in FIG. 18A, the value indicated by the line V181 represents the output signal from the light receiving element 130 in the first lighting operation, the value indicated by the line V182 represents the output signal from the light receiving element 130 in the second lighting operation, and the value indicated by the line V183 corresponds to the difference between the output signal from the light receiving element 130 in the first lighting operation and the output signal from the light receiving element 130 in the second lighting operation. In such a result, although the difference represented by the line V183 should monotonically vary as the angular position of the target object Ob moves, in fact, in the case in which the reflected light L3 enters the light receiving surface 130a of the light receiving element 130 in the direction significantly tilted from the normal direction, the level of the output signal from the light receiving element 130 is extremely low, and at the same time, the difference becomes to fail to show the monotonic variation.
It should be noted that by multiplying the result shown in FIG. 18A by the sensitivity shown in FIG. 17B, the result shown in FIG. 18A is converted into the result shown in FIG. 18B. In FIG. 18B, the value indicated by the line V186 represents the result of the multiplication of the output signal from the light receiving element 130 in the first lighting operation and the sensitivity, the value indicated by the line V187 represents the result of the multiplication of the output signal from the light receiving element 130 in the second lighting operation and the sensitivity, and the value indicated by the line V188 corresponds to the difference from the output signal after the conversion. As shown in FIG. 18B, by performing the conversion using the sensitivity, the level of the output signal from the light receiving element 130 is set to be sufficiently high and the difference shows the monotonic variation in a large angular range even in the case in which the reflected light L3 enters the light receiving surface 130a of the light receiving element 130 in the direction significantly tilted from the normal direction. Therefore, it can be said that the abnormal variation shown in FIG. 18A is caused by the directivity in sensitivity of the light receiving element 130, and the incident light intensity itself to the light receiving element 130 is appropriate irrespective of the incident angle.
Therefore, although it results that it is sufficient to arrange the light receiving element 130 so as to surround the detection space 10R in order for solving the problem caused by the directivity in sensitivity of the light receiving element 130, such a configuration makes the device grow in size, and is therefore difficult to be put into practice.